It is known to provide projection displays having high intensity discharge lamps. There are a number of major types of High Intensity Discharge (HID) lamp technologies, including for example ultra high pressure mercury, short arc xenon and short arc metal halide types. HID lamps typically have a quartz glass lamp bulb and tungsten electrodes. The quartz glass bulb is the discharge vessel containing the electrode system and is filled with a gas or gas mixture. To start an HID lamp an ignition voltage (a series of high-frequency pulses typically in the 30 kV range) is required. One electrode has the function of cathode; the other has the function of anode. When an electric field is passed between the electrodes, electrons are emitted from the cathode by thermionic emission; the anode has to collect the electrons and has to dissipate more power than the cathode (negative workfunction). Both AC and DC operated HID lamps are known. Flicker is less of a problem with DC operated lamps.
U.S. Pat. No. 4,914,356 discloses that the standard practice to drive gas discharge lamps has been to supply the gas discharge lamp with alternating current (AC), and to use a series reactance to limit the current to the lamp. However, it is also possible to operate gas discharge lamps with direct current (DC).
DC operation of HID lamps has been used for example when only a DC supply is available and/or to reduce flicker. DC operation reduces life time significantly due to electrode erosion, see for example SID 2004 digest, page 946-949, Holger Moench et al. in “Controlled electrodes in UHP lamps”, which states that for AC lamps, if electrodes and operating conditions are designed correctly, there is a balance between evaporation of electrode material and ion back-transport, which keeps the electrodes stable for over 10000 hours. This regenerative mechanism for lamp electrodes works well only for AC lamps, as in case of DC operation any ion current is directed towards the cathode only. Therefore, the hot anode quickly burns back. Thus, while AC lamps can be regenerated for thousands of hours, DC lamp electrodes are degrading quickly during lamp life.
The above difficulties with DC lamps have resulted in specialised DC designs being developed which differ significantly from AC designs. One design strategy to try and reduce the negative effect of evaporation of electrode material from one electrode to the other is to change polarity at each switch on. However, this requires symmetrical electrodes. It is preferred to use asymmetrical electrodes, i.e. to make the anode much larger in size and diameter (to keep its temperature under control) compared to the cathode, and therefore changing polarity on switch on is not an option. Although it is known to compensate for loss of electrode material, no electronic ballast technique is known to change the fundamental life time of DC operated lamps, especially ones with asymmetrical electrodes.
Furthermore, In addition, U.S. Pat. No. 4,373,146 states that electronic ballasts using DC lamp operation or phase control at low frequencies are not as desirable from a lamp life point of view.
Ways of operating AC HID lamps have been disclosed in U.S. Pat. No. 5,608,294 and U.S. Pat. No. 6,779,896. These ways of operating relate to reducing flicker phenomena. However, as the flicker phenomena of DC HID lamps differs fundamentally from those of AC lamps, such methods have never been considered for DC lamps.
The above design considerations have resulted in efficient AC operated HID lamps even for low power ratings such as 300 watt, with arc lengths 1.5 mm or less. Reducing arc length worsens the lifetime problem. Increasing the arc length inceases the operation voltage. Typically, DC operated lamps have been used for the higher power ratings, i.e. e.g. with arc lengths bigger than 2.5 mm and above 300 watt. There remains a need for improved DC devices.